In the oil and gas industry, wells are drilled from offshore platforms located in water which may exceed one thousand feet in depth. To minimize the cost of producing oil and gas from a reservoir, drilling contractors typically drill a group of wells from a single platform. The wells can be "kicked out" from the base of the platform by using directional drilling techniques to reach locations in the underground reservoir at a distance from the base of the platform.
Each well is connected to a riser which extends from the sea floor to a deck of the offshore platform. Drilling rigs, which comprise a drilling derrick, a supporting structure, and drilling machinery, are used to drill the wellbore for each well. Once a wellbore has been drilled, the drilling rig must be moved to a new location on the deck of the platform so that a new well can be drilled. This process is repeated until all of the wells have been drilled from the platform. On large offshore platforms, dozens of wells may be drilled from the same platform deck.
The drilling rig and associated equipment is extremely heavy and is not easily transported from one well to another. Typically, the drilling rigs are supported on parallel beams known as "capping beams." The capping beams are fastened to the platform deck and are usually raised slightly above the platform deck. To move the drilling rig to a new location, the drilling rig is skidded along the capping beams by using large hydraulic cylinders which are anchored to the capping beams. In operation, one end of each hydraulic cylinder is anchored to a capping beam, and the hydraulic cylinders are simultaneously extended to their full stroke to move the drilling rig along the capping beams. The anchors are then disconnected, the cylinders are retracted, and the sequence is repeated until the drilling rig is located in the desired location.
Several types of devices are used to anchor the hydraulic cylinders to the capping beams. Claw type anchors positively engage the capping beams by using "dogs" or "claws" which are inserted into slots cut into the capping beams. "Grippers" are anchors which use a large clamping force to grip the capping beams. The clamping force must develop sufficient friction force between the gripper and the capping beam to prevent the anchor from slipping when the hydraulic cylinder is actuated to move the drilling rig.
Claw type anchors are less expensive than grippers when the capping beam is relatively short. On long capping beams, the cost increases with the number of slots which must be cut into the beam. For that reason, grippers are typically used for longer capping beams on platform decks which employ numerous wells. However, certain disadvantages are associated with gripper type anchors. For example, the frames of grippers must be sufficiently large to handle the bending loads produced by the clamping mechanism. Various types of grippers use hydraulic cylinders actuated wedge clamps, or hydraulic actuated lever clamps to generate the large clamping force necessary to hold the anchor. The frames necessary to hold these gripper type anchors are heavy and occupy scarce space on the deck of the offshore platform.
For these reasons, there is a need for an improved anchor which can selectively engage and disengage a beam such as a capping beam. On an offshore platform, the anchor should efficiently maximize the force engaging the beam to prevent movement of the anchor while the drilling rig is being moved.